One problem associated with the cleaning of crankcase gases deals with maintaining a desired gas pressure in the crankcase. There is a risk of pumping too much crankcase gas out of the crankcase by means of the centrifugal separator, wherein the gas pressure in the crankcase may decrease to an unallowably low level, and for instance be significantly below the surrounding atmospheric pressure. Such a low gas pressure in the crankcase can also arise when the combustion engine runs at an idle speed or at a very low rotational speed. This can have a negative influence on the gaskets sealing the crankcase. In addition, contaminated air can be sucked into the crankcase and give rise to damage. In certain situations there is also a risk that the centrifugal separator would not be able to pump away a sufficient quantity of crankcase gas so that the gas pressure in the crankcase increases to an unallowably high level, significantly above the surrounding atmospheric pressure. This can also have a negative influence on the gaskets and in some cases lead to leakage of crankcase gas from the crankcase of the combustion engine. It is to be noted that combustion engines of various types and from various manufactures are operated at different gas pressures in the crankcase. In certain combustion engines, it is desirable to maintain an overpressure in the crankcase, i.e. a gas pressure which is somewhat higher then the surrounding atmospheric pressure, whereas in other combustion engines it is desirable to maintain a subpressure in the crankcase, i.e. a gas pressure which is somewhat lower then the surrounding atmospheric pressure. The desired level of the gas pressure in the crankcase depends on various factors, for instance the design of the above-mentioned crankcase gaskets.
WO99/56883 (Alfa Laval AB) discloses a device for cleaning of crankcase gases from a combustion engine. The device comprises an inlet conduit, which is connected to a crankcase of the combustion engine, a centrifugal separator, which receives crankcase gases from the crankcase via the inlet conduit and which comprises a centrifuge rotor for centrifuging the crankcase gases, a drive device, which drives the centrifuge rotor and, an outlet conduit for discharge of cleaned crankcase gases from the centrifugal separator. The known drive device is intended to be operated by means of any pressurised medium, such as air, oil, cooling water or fuel. Also other examples of the driving are defined. This document also discloses various embodiments of the centrifuge rotor. The known device is also adapted to enable discharge of the cleaned crankcase gases from the centrifugal separator to the surroundings or to the air intake of the combustion engine.
SE-522 473 (Alfa Laval AB) discloses another device for cleaning of crankcase gases from a combustion engine. The device comprises an inlet conduit, which is connected to a crankcase of the combustion engine, a centrifugal separator, which receives crankcase gases from the crankcase via the inlet conduit and which comprises a centrifuge rotor for centrifuging the crankcase gases, a separate electric motor driving the centrifuge rotor, and an outlet conduit for discharging cleaned crankcase gases from the centrifugal separator. This device also comprises a control equipment for controlling the rotation speed of the electric motor and thus of the centrifuge rotor. The purpose of this controlling is in the first place to provide an efficient cleaning of the crankcase gases. The known control equipment is adapted to control and change the rotation speed of the electric motor depending on a variety of different parameters.
WO2004/001200 (Alfa Laval Corporate AB) discloses a further device for cleaning of crankcase gases from a combustion engine. The device is similar to the device disclosed in the above-mentioned SE-522 473 and comprises a control equipment for controlling the rotation speed of the electric motor and thus of the centrifuge rotor. The purpose of this controlling is in the first place to provide an efficient cleaning of the crankcase gases. The known control equipment is adapted to control and change the rotation speed of the electric motor depending on a variety of different parameters in such a way that the gas pressure in the crankcase is maintained at a desired level.